It has been found desirable to package food and other products which require hermetic seals in peelable seal packages which essentially comprise a pair of superimposed sheets with the product therebetween, said sheets being sealed together to provide a seal strong enough to withstand abuse and resist separation and leakage. At the same time, it is desirable to provide for peeling the sheets apart to make the article within the package readily accessible with a minimum of effort and without the necessity of cutting or tearing the package. In one such package, a pocket or cavity is formed in one of the sheets by drawing the sheet into a mold for the retention of the product and the other sheet is used as a closure and is sealed peripherally around the cavity to a relatively large flange area. In another such package, the product is placed on an impervious backing member or sheet within a vacuum chamber and a formable film is molded around the product and sealed to a large area of the backing member to create a vacuum skin package. Such packages must protect the contents against the intrusion of moisture, air, and other contaminates and accordingly, must be impervious and have effective seals yet must be readily peelable without tearing or rupturing the packaging materials which may result in premature destruction of the package and inadvertent contamination of the contents. Furthermore, as a simple matter of convenience it is desirable that the package be openable by manually pulling apart the film sheets instead of relying upon a knife, scissors, or tear tab to open the package. Accordingly, it is one object of the present invention to provide a package which may be peeled open by only manual force.
In the prior art a number of package configurations and materials have been used in order to achieve a readily peelable seal. In some instances, an intermediate polymeric material is provided between the sheets to serve as an adhesive or as a sealant between the film sheets forming the package. However, each material layer which is added to the sheets comprising the package increases the package cost and difficulty of manufacturing. Accordingly, it is another object of the present invention to provide a package which does not require the addition of a peelable adhesive coating between the film sheets.
Also in the prior art, cross-linking by irradiation has been used to enhance the sealing properties of the films comprising a package. For example, in U.S. Pat. No. 3,106,441 which issued on Oct. 8, 1963 to J. W. Harrison et al irradiation of the seal area to greatly improve seal strength is taught. In another patent, U.S. Pat. No. 3,022,543 which issued to W. G. Baird et al on Feb. 27, 1962, a thin peelable cover of irradiated, biaxially oriented film is shown sealed to an unirradiated, thermoformed container of relatively thick plastic material. The Baird et al patent also discloses biaxially oriented, irradiated polyethylene film sealed to itself. In these prior art disclosures, the polyolefin material is either irradiated and biaxially oriented or is irradiated after the final sealing has taken place whereas it is another object of the present invention to provide a package which is made from unoriented, cross-linked polyolefin materials in which both films are cross-linked prior to sealing but not irradiated afterwards thereby forming a package having more easily peelable seals.
These and other objects will be readily apparent from the summary of the invention, drawings, and detailed description which follow: